Voltage boosting circuit

Abstract: A voltage boosting circuit for boosting a supply voltage VCC supplied from a system to a desired boosting voltage VPP level is described. The voltage boosting circuit includes a transmission transistor 26 formed by a triple-well process. The transmission transistor 26 has bipolar characteristics and operates as a bipolar diode.

Description: The present invention relates to a semiconductor memory device, and more particularly to a voltage boosting circuit for boosting a supply voltage supplied from a system to a desired boosting voltage level. The term "voltage boosting circuit" has the same meaning in this field as "boosting circuit", "boosting voltage generating circuit", "bootstrap circuit", etc. In a dynamic RAM consisting of CMOS transistors, there occurs a voltage drop by the threshold voltage of the MOS transistor in the effective potential while the potential is transmitted through the channel region of the MOS transistor. This inevitable voltage drop becomes an obstacle to the accurate reading or writing of data as well as the loss of data. To solve such a problem, a voltage boosting circuit for raising the voltage level has come to be used. A voltage boosting circuit comprising bipole, diodes is known from U. TIETZE-CH. SCHENK, Halbleiterschaltungstechnik, Springer Verlag Berlin, 1974, pages 35-36. DE-A-4 237 589 discloses a voltage boosting circuit according to the preamble of claim 1, having precharge and transmission MOS transistors. As further examples of such a voltage boosting circuit, there are Korean patent No. 91-19740; a paper entitled "A 35ns 64Mb DRAM Using On-Chip Boosted Power Supply", 1992 Symposium on VLSI Circuits Digest of Technical Papers, pp.64-65 and U.S. patent No. 4,704,706 granted to Japan Fujitsu Co. Fig. 1 schematically shows a characteristic portion of a voltage boosting circuit which is generally well known in the art and described in the aforementioned documents. An input node 2 receives an oscillating signal OSC generated from an oscillator (not shown). A pumping capacitor 4 has one electrode terminal coupled to the input node 2 and the other electrode terminal coupled to a pumping node 8. A pumping capacitor 6 has one electrode terminal coupled to the input node 2 and the other electrode terminal coupled to a pumping node 10. A transmission transistor 12 with the gate and drain connected to the pumping node 8 and 10 respectively generates a boosting voltage VPP. Although not shown in Fig. 1, there is further provided a precharge circuit for precharging the pumping nodes 8 and 10 to a supply voltage VCC level. The construction of Fig. 1 is known in the art as a charge pump circuit. The oscillator operates when the boosting voltage VPP is lowered to a level lower than a normal level by an internal circuit during power-up of a chip and active cycle. If the oscillating signal OSC is supplied to the input node 2, the pumping capacitors 4 and 6 boost the pumping nodes 8 and 10 to about twice the supply voltage VCC. A voltage charged to the pumping node 10 is generated through the channel of the transmission transistor 12 as the boosting voltage VPP. The circuit of Fig. 1 is driven by the oscillator using as source voltage the supply voltage VCC, so that the boosting voltage VPP level of 2VCC-VT (where VT is the threshold voltage of the transmission transistor 12) can be obtained. The pumping nodes 8 and 10 are initially precharged to the supply voltage VCC level. The voltage boosting circuit of Fig. 1 is formed by a typical CMOS manufacturing process. The circuit of Fig. 1 has the problem of pumping efficiency in that the transmission transistor 12 is an NMOS transistor formed by the CMOS manufacturing process. As is well known to those skilled in the art, the device characteristics of MOS transistors are such that the body effect increases with the increase in voltage level across its source and drain. It is apparent that the body effect increases further since the size of each device is shortened and space thereof becomes narrower with increasing integration of semiconductor memory devices. The boosting voltage circuit of Fig. 1 has a fundamental problem that the pumping efficiency is lowered by the device characteristics of the MOS transistor, i.e. of the transmission transistor, rather than by circuit design. It is an object of the invention to provide a voltage boosting circuit with improved pumping efficiency. It is another object of the invention to provide a voltage boosting circuit for generating a boosting voltage at high speed. Accordingly, the present invention provides a voltage boosting circuit for use in a semiconductor memory device, as defined by claim 1. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example only, to the attached diagrammatic drawings, in which: Fig. 1 is a schematic diagram illustrating a characteristic portion of a conventional voltage boosting circuit; Fig. 2 is an equivalent circuit diagram illustrating a schematic construction of a voltage boosting circuit according to the present invention; Fig. 3 is a sectional view of the circuit of Fig. 2; Fig. 4 is a circuit diagram illustrating one embodiment of a voltage boosting circuit constructed in accordance with Figs. 2 and 3; and Fig. 5 is a waveform chart illustrating the rising slope of a resultant boosting voltage VPP of the circuit of Fig. 4. Referring to Fig. 2, a voltage boosting circuit according to the present invention uses a bipolar transistor 26 as a transmission transistor for generating a boosting voltage VPP. In the present description, the terms "bipolar transistor", "bipolar diode" and "bipolar transmission transistor" are used interchangeably. As is well known to those skilled in the art, a bipolar transistor is a current controlled device, while a MOS transistor is a voltage controlled device. The bipolar transistor has advantages in that the switching speed is improved and the driving force increases. The transmission transistor 26 made of the bipolar transistor has sophisticated device features as the voltage boosting circuit, such as the improvement of the pumping efficiency, the high speed boosting operation to a desired boosting level, and the prevention of body effect. Fig. 3 is a sectional view of the circuit of Fig. 2. The bipolar transistor 26 is formed by a typical CMOS triple-well manufacturing process. The process will now be described in brief. An N-well 30 is formed on a P-type substrate 28. A P-well 32 is formed at the middle portion within the N-well 30. Within the N-well 30 except where the P-well 32 is formed, an n+ layer 38 is formed by implantation of an n+ impurity. A p+ layer 40 and an n+ layer 42 are formed within the P-well 32 by implantation of p+ and n+ impurities, respectively. Thus the structure of the bipolar transistor 26 shown in Fig. 2 is completed. The n+ layers 34 and 36 formed by implantation of the n+ impurity on the P-type substrate 28 and a gate 35 constitute a pumping capacitor 22 of Fig. 2. Since the structure of Fig. 3 is easily achieved using a typical manufacturing process, details such as a mask process, a diffusion process, etc. are omitted. The n+ layers 34 and 36 are connected to an input node 20 and pump the gate 35 formed thereon in response to a voltage across the input node 20, thereby pumping a pumping node 24 coupled to the gate 35. It should be noted that the pumping node 24 is commonly connected to the n+ layer 38 within the N-well 30 and the p+ layer 40 within the P-well 32, and details thereof will be discussed later on. A line to which the boosting voltage VPP is applied is connected to the n+ layer 42 within the P-well 32. It will be readily appreciated that the bipolar transistor 26 operates through the P-well 32 between the pumping node 24 and the boosting voltage VPP line. To pump the boosting voltage VPP through the bipolar transistor 26, the pumping node 24 should be isolated from the P-type substrate 28. Otherwise, pumping voltage charged to the pumping node 24 is discharged through the substrate 28. One skilled in the art will appreciate the need for the formation of the bipolar transistor within the N-well 30 to prevent the pumping voltage from discharging. The reason why the pumping node 24 is commonly connected to the n+ layer 38 within the N-well 30 and the p+ layer 40 within the P-well 32 will now be described. If the pumping node 24 cannot supply the pumping voltage to the N-well 30 through the n+ layer 38, that is, if the pumping node 24 supplies the pumping voltage only to the p+ layer 40 within the P-well 32, there is formed a P-N forward bias. Accordingly, a high voltage applied to the P-well 32 is drained to the N-well 30. Then the n+ layer 42 of the P-well 32 is not pumped to a desired level. Hence, the high voltage should be supplied from the N-well 30. To overcome this problem, it is desirable that the pumping node 24 is connected to the n+ layer 38 within the N-well 30. Meanwhile, it is desirable that the P-type substrate 28 is connected to a ground voltage GND or a substrate voltage VBB in order to prevent a P-N forward bias. In a bipolar transistor 26 with the above-mentioned triple-well structure, there are no current variations even though the boosting voltage VPP is raised, thereby minimizing the time required for pumping the boosting voltage VPP to a desired high voltage. Therefore the pumping efficiency increases and the device operating characteristics of the voltage boosting circuit are ensured. Fig. 4 is one embodiment of the voltage boosting circuit constructed in accordance with Figs. 2 and 3. The voltage boosting circuit of Fig. 4 shows the construction of a charge pump circuit. The voltage boosting circuit performs a double pumping operation in response to logic change of a triggered oscillating signal OSC from an oscillator. An input node 44 receives the oscillating signal OSC generated from the oscillator (not shown). A first inverter 46 has an input terminal connected to the input node 44. A first pumping capacitor 48 having both terminals of the electrode connected between the first inverter 46 and a first pumping node 50 pumps the first pumping node 50 in response to the voltage level of the output signal of the first inverter 46. A first precharge transistor 52 precharges the first pumping node 50 to a voltage level VCC-Vtn. A first bipolar transistor 54 of an NPN transistor has the base and collector commonly connected to the first pumping node 50 and the emitter connected to a boosting node 72 for generating the boosting voltage VPP. A second inverter 60 has an input terminal connected to the input node 44, and a third inverter 62 has an input terminal connected to the output terminal of the second inverter 60. A second pumping capacitor 64 having both terminals of the electrode connected between the third inverter 62 and a second pumping node 66 pumps the second pumping node 66 in response to the voltage level of the output signal of the third inverter 62. A second precharge transistor 68 precharges the second pumping node 66 to the voltage level VCC-Vtn. A second bipolar transistor 70 of an NPN transistor has the base and collector commonly connected to the second pumping node 66 and the emitter connected to the boosting node 72. A third precharge transistor 56 having the channel connected between a supply voltage VCC terminal and the first pumping node 50 and having the gate connected to the second pumping node 66 precharges the first pumping node 50 to the supply voltage VCC level. A fourth precharge transistor 58 having the channel connected between the second pumping node 66 and the supply voltage VCC terminal and having the gate connected to the first pumping node 50 precharges the second pumping node 66 to the supply voltage VCC level. The structure of Fig. 4 has a feature that the charge pump circuit uses a bipolar transistor as the transmission transistor. The operation of the circuit of Fig. 4 will now be discussed. After power-up of a chip, when the voltage boosting circuit of Fig. 4 is a disabled state, by maintaining the boosting voltage VPP at a desired level or before the chip is enabled, the pumping nodes 50 and 66 are precharged to the voltage level VCC-Vtn by the precharge operation of the first and second precharge transistors 52 and 68. Where Vtn is the threshold voltage of the precharge transistor 52 or 68 of an NMOS transistor. Thereafter, if the chip is enabled or the boosting voltage VPP is lowered to a level lower than the desired level, the voltage boosting circuit of Fig. 4 is enabled. Then the oscillating signal OSC is supplied to the input node 44 as a square wave having a constant period. The detailed description is as follows. First, if the oscillating signal OSC supplied to the input node 44 is raised from logic "low" to logic "high", the first inverter 46 generates logic "low". There is no pumping operation through the first pumping capacitor 48 and the first pumping node 50 maintains its precharge level. The second and third inverters 60 and 62 generate logic "low" and "high", respectively. The second pumping capacitor 64 boosts the second pumping node 66 to twice the voltage level VCC-Vtn. The boosted voltage level of the second pumping node 66 is transmitted through the second bipolar transistor 70 to the boosting node 72 to raise the boosting voltage VPP. The second bipolar transistor 70 is a device formed by the structure of Fig. 3. During the charge sharing operation of the second pumping node 66 and the boosting node 72 through the second bipolar transistor 70, the second bipolar transistor 70 has the device characteristics described in reference to Fig. 3. Thus the high voltage charged to the pumping node 66 is transmitted to the boosting node 72 at high speed, and the body effect does not take place even though the boosting node 72 becomes a high voltage level. The emitter of the first or second bipolar transistor 54 or 70 corresponds to the n+ layer 42 within the P-well 32 of Fig. 3, the base of which is the p+ layer 40 within the P-well 32 of Fig. 3, and the collector of which is the n+ layer 38 within the N-well 30 of Fig. 3. Hence, it will be readily understood that the body effect does not occur even though the boosting node 72 is raised to the high voltage. The voltage level 2(VCC-Vtn) pumped at the second pumping node 66 fully turns on the channel of the third precharge transistor 56 to precharge the first pumping node 50 to the supply voltage VCC level. Second, if the oscillating signal OSC supplied to the input node 44 is lowered from logic "high" to "low", the first inverter 46 generates logic "high". The first pumping capacitor 48 pumps the first pumping node 50 from the previous precharge level VCC (this level is obtained through the third precharge transistor 56 in the aforementioned process) to twice the precharge level VCC. The boosted voltage level of the pumping node 50 is transmitted to the boosting node 72 via the first bipolar transistor 54 to raise the boosting voltage VPP to a higher level than the previous boosting voltage level. The first bipolar transistor 54 is also formed by the structure of Fig. 3 and the device characteristics described in reference to Fig. 3 are maintained during the charge sharing operation of the first pumping node 50 and the boosting node 72 through the first bipolar transistor 54. Therefore, the high voltage charged to the pumping node 50 is transmitted to the boosting node 72 at high speed, and the body effect caused by the raised voltage of the boosting node 72 is prevented. On the other hand, the second and third inverters 60 and 62 generate logic "high" and "low" respectively. There is no pumping operation of the second pumping node 66 through the second pumping capacitor 64. The voltage level 2VCC pumped at the first pumping node 50 fully turns on the channel of the fourth precharge transistor 58 to precharge the second pumping node 66 to the supply voltage VCC level. Consequently, the efficiency of the next pumping operation is raised. If the oscillating signal OSC is again raised from logic "low" to "high", the aforementioned first process is repeated. The precharge level of the first pumping node 50 becomes the supply voltage VCC level and then the pumping operation proceeds. A succession of processes is repeated until the boosting voltage VPP is raised to the desired boosting voltage level, and through these processes the boosting voltage VPP is raised to the prescribed voltage level. Fig. 5 shows a rising slope of the boosting voltage VPP resulting from the operating characteristics of Fig. 4. As shown, the inventive voltage boosting circuit has a faster set-up time to raise the boosting voltage VPP to a desired boosting voltage level (about twice the supply voltage VCC) than the conventional circuit. It is apparent that these characteristics can be achieved by the first and second bipolar transistors 54 and 70 of Fig. 4 constructed in accordance with the structure of Fig. 3. As described above, the voltage boosting circuit embodying the present invention raises the boosting voltage VPP to a desired boosting voltage level by achieving a charge pump circuit having a bipolar transistor. Even if the boosting voltage VPP across the boosting node is raised, the boosting efficiency is raised since there are no current variations flowing into the boosting node. Further, body effect which increases in proportion to the raised boosting voltage in a typical voltage boosting circuit is prevented. The foregoing description shows only a preferred embodiment of the present invention. Various modifications are apparent to those skilled in the art without departing from the scope of the present invention.

Claims: A voltage boosting circuit for use in a semiconductor memory device, comprising: an input node (44) for receiving an oscillating signal; a pumping node (50; 66) connected to precharge MOS transistors (52, 56; 68, 58) and adapted to be precharged to a prescribed voltage level; a boosting node (72); a pumping capacitor (48; 64) having its electrode terminals connected between said input node and said pumping node, said pumping capacitor being adapted to pump said pumping node in response to the oscillating signal; characterised in that a bipolar transistor (54; 70) is connected between said pumping node and said boosting node for transmitting the voltage charged from said pumping node to said boosting node, wherein a base node and a collector node of said bipolar transistor are commonly connected to said pumping node. The voltage boosting circuit as set forth in claim 1, wherein said bipolar transistor is an NPN bipolar transistor. A voltage boosting circuit according to any of claims 1 or 2, wherein said bipolar transistor comprises: a first well (30) of a second conductivity type formed on a substrate (28) of a first conductivity type; a second well (32) of said first conductivity type formed within said first well; a first diffusion layer (38) of said second conductivity type formed within said first well but not within said second well, said first diffusion layer being connected to a first electrode terminal (35) of said pumping capacitor (22); a second diffusion layer (40) of said first conductivity type formed within said second well and connected to said first electrode terminal of said pumping capacitor; and a third diffusion layer (42) of said second conductivity type formed within said second well and connected to said boosting node (72); whereby said bipolar transistor generates a boosting voltage pumped through said pumping capacitor to said boosting node (72). The voltage boosting circuit as set forth in claim 3, wherein said first type is of a P-type, and wherein said second type is of an N-type. A voltage boosting circuit according to any preceding claim, including an oscillator for generating an oscillating signal and comprising: inverter means (46, 60, 62) for receiving said oscillating signal through the input node and for outputting first and second oscillating output signals, the first oscillating output signal being substantially an inversion of the second oscillating output signal; a first pumping capacitor (48) having its electrode terminals connected between said inverter means and a first pumping node (50) and adapted to pump said first pumping node in response to said first oscillating output signal; a first precharge transistor (52) for precharging said first pumping node to a level corresponding to a supply voltage; a first bipolar device comprising a first bipolar transmission transistor (54) with its base and collector commonly connected to said first pumping node and its emitter connected to the boosting node; a second pumping capacitor (64) having its electrode terminals connected between said inverter means and a second pumping node (66) and adapted to pump said second pumping node in response to said second oscillating output signal; a second precharge transistor (68) for precharging said second pumping node to said level corresponding to said supply voltage; and a second bipolar device comprising a second bipolar transmission transistor (70) with its base and collector commonly connected to said second pumping node and its emitter connected to said boosting node. The voltage boosting circuit as set forth in claim 5 in which said inverter means comprises: a first inverter (46) with an input terminal connected to said input node and an output terminal adapted to provide said first oscillating output signal; a second inverter (60) with an input terminal connected to said input node; and a third inverter (62) with an input terminal connected to the output terminal of said second inverter and an output terminal adapted to provide said second oscillating output signal. The voltage boosting circuit as set forth in claim 5 or claim 6, further comprising: a third precharge transistor (56) with the channel connected between said first pumping node and a supply voltage terminal and with the gate connected to said second pumping node, said third precharge transistor precharging said first pumping node to a level corresponding to a full supply voltage level; and a fourth precharge transistor (58) with the channel connected between said second pumping node and said supply voltage terminal and with the gate connected to said first pumping node, said fourth precharge transistor precharging said second pumping node to said level corresponding to said full supply voltage. A voltage boosting circuit according to claim 1 including an oscillator for generating an oscillating signal and comprising: inverter means (46, 60, 62) for receiving said oscillating signal through an input node and for outputting first and second oscillating output signals, the first oscillating output signal being substantially an inversion of the second oscillating output signal; a first pumping capacitor (48) having its electrode terminals connected between said inverter means and a first pumping node (50) and adapted to pump said first pumping node in response to said first oscillating output signal; means (52, 56) for precharging said first pumping node to a level corresponding to a supply voltage; a first bipolar transmission transistor (54) for generating a boosting voltage pumped through said first pumping capacitor to the boosting node; a second pumping capacitor (64) having its electrode terminals connected between said inverter means and a second pumping node (66) and adapted to pump said second pumping node in response to said second oscillating output signal; means (58, 68) for precharging said second pumping node to said level corresponding to said supply voltage; and a second bipolar transmission transistor (70) for generating a boosting voltage pumped through said first pumping capacitor to said boosting node.

Inventors: CHOI HOON, CHOI, HOON
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1994
Main classification: G11C5
IPC codes: G11C11, H01L27, H01L21, H03K5, H02M3, G11C5